The present invention generally relates to a modulator for modulating a carrier in a code division multiple access system, and particularly to a multicarrier quadrature amplitude modulator.
Over the past several years, code-division multiple access (CDMA) systems have gained widespread interest in the mobile wireless communications. A wide band code division multiple access (WCDMA) uses a wider channel compared to a narrow band CDMA channel which improves frequency diversity effects and therefore reduce the fading problems. Due to resistance to multipath fading and other advantages such as increased capacity and soft-handoff, the WCDMA was selected by European Telecommunications Standars Institute (ETSI) for wide band wireless access to support third-generation mobile services. This technology is optimized to allow very high-speed multimedia services such as full-motion video, internet access and video conferencing.
In a WCDMA system one or more carriers may be used. In the above mentioned European WCDMA system, four QAM modulated carrier frequencies are needed to generate in a base station. A straightforward solution is to modulate separately each of the carriers whereupon the carriers are combined before feeding to an antenna. In a consequence of this, the number of processing branches is equal to the number of carriers.
FIG. 1 depicts a prior art multicarrier transmitter for generating four modulated carriers. Each carrier is processed separately in an own branch. All the branches are similar so that operation of the upper branch will be described only. Modulating digital I and Q base band signals are produced in an digital base band I-Q modulator 11 which is well known in the art. Then, each of the digital signals is separately converted to an analog signal with DA converters 12 and 14, the outputs signals of which being applied to low pass filters 13 and 15. The filtered I signal is mixed with a carrier frequency signal obtained from local oscillator LO1. Accordingly, the filtered Q signal is mixed with the 90xc2x0 phase shifted carrier signal. Both carrier frequency signals are combined in summing element 19, and the combined signal is firstly filtered in the band pass filter, then amplified in power amplifier 111. Finally, the amplified signal is fed to element 112 which combines the signal with those obtained from the other branches.
The arrangement for producing four carriers as shown in FIG.1 requires four pair of D/A-converters, four pair of low-pass filters, four analog I/Q modulators, four local oscillators (LO1, LO2, LO3, and LO4) and four power amplifiers.
FIG. 2 is a block diagram of another conventional QAM modulator. Here, modulation is carried out digitally. Sine and cosine intermediate frequency signals are generated by a numerically controlled oscillator 21. Input to the oscillator is xe2x80x9ccarrier frequencyxe2x80x9d which is a frequency control word. The phase value is generated by using the modulo 2j overflowing property of a j-bit phase accumulator. The phase accumulator addresses the sine/cosine Read Only Memories (ROMs) which convert phase information into values of a sine/cosine wave. Phase accumulator 22 controls sine and cosine read only memories 23, 24 to output digital sine(xcfx89NCO) and cosine(xcfx89NCO) signals. The cosine intermediate frequency signal is multiplied with the I data which is before multiplying filtered both in root raised cosine filter 25 and interpolation filters 26. The root raised cosine filter reduces the transmitted bandwidth, which means that more channels can be occupied in the frequency band. Furthermore, after this filter the signal fulfils the Nyquist criterion (no intersymbol interference). Accordingly, the sine signal is multiplied with the Q data which is before multiplying filtered both in root raised sine filter 27 and interpolation filters 28. Then the modulated sine and cosine intermediate frequency signals are combined in combiner 29. Note, that instead of the numerically controlled oscillator 21, direct digital synthesis DDS may be used as well.
The output of the modulator in FIG. 2 is:
s(n)=I(n)cos(xcfx89NCOt(n))+Q(n)sin(xcfx89NCOt(n))xe2x80x83xe2x80x83(1)
where xcfx89NCO is the output frequency of the numerically controlled oscillator, and I(n), Q(n) are pulse shaped and interpolated quadrature data symbols.
The pre-equalizer could be used to compensate for the sinx/x roll-off function inherent in the sampling process of the digital-to-analog conversion. Furthermore, distortions in the phase and magnitude of response of the analog filters could be partly pre-compensated by the pre-equalizer. The analysis and compensation of the distortions from analog filters are left out of the scope of this paper
FIG. 3 is a table containing specifications for a base station transmitter in the European WCDMA system. As seen from the table, four carries are used with carrier spacing 5 MHz.
FIG. 4 is another way to build a conventional QAM modulator with complex outputs. In comparison with the modulator of FIG. 2, this modulator uses two additional multipliers and one additional adder to produce both I and Q output signals, which are:
IOUT=I(n)cos(xcfx89NCOt(n))+Q(n)sin(xcfx89NCOt(n)),
QOUT=Q(n)cos(xcfx89NCOt(n))xe2x88x92I(n)sin(xcfx89NCOt(n)),
This structure requires two adders, four multipliers, and sine/cosine memories.
A drawback of the analog multicarrier modulator is the need of huge amount of analog components. In addition, many of them require production tuning including complexities of adjusting the dc offset, the phasing, and the amplitude levels between the in-phase and quadrature phase. Tuning is an expensive part of manufacturing. Furthermore, the analog I-Q modulator causes most of error vector magnitude (EVM) in the conventional systems. The EVM is defined as the difference between ideal vector convergence point and transmitted point in the signal space. EVM is defined as r.m.s. value of the error vectors in relation to the magnitude at a given symbol
A drawback of the conventional digital QAM modulators are that they need several multipliers and sine/cosine ROMs, which cannot be efficiently implemented with field programmable gate arrays (FPGA).
An objective of the present invention is to provide a digital QAM modulator without need of multipliers and sine/cosine memories and which is easy to implement with field programmable gate arrays.
The objective is achieved by replacing the circuitry which carries out the circular rotation of [I(n), Q(n)]T performed in all the QAM modulators, with a CORDIC algorithm. Said algorithm is an iterative algorithm for computing many elementary functions.
The invented CORDIC based QAM modulator is comprised of a plurality of digital rotation stages coupled in sequence. The first stage receives the IIN and QIN data streams and the last stage outputs a vector comprising of the orthogonal I and Q components of a digital intermediate frequency signal. Each of the rotation stages includes pipeline registers and adding/subtracting elements for rotating an input vector applied to the stage at a predetermined elementary rotation angle, said input vector being the output vector of the previous digital rotation stage.
The modulator further includes an angle computation block for counting the elementary rotation angles for the rotation stages. The angle computation block is connected to the phase accumulator which receives a frequency control word and based on that word generates an input word applied to the angle computation block.
Implementation of the modulator consists an array of interconnected adder/subtractors. This structure is easy to pipeline, therefore the clock frequency requirement of the QAM modulator can be reached in FPGA design.
The invented multicarrier QAM modulator does not use an analog I/Q modulator, therefore, the difficulties of adjusting the dc offset, the phasing and the amplitude levels between the in-phase and quadrature phase signal paths are avoided.
The CORDIC based QAM modulator has about same logic complexity as the two multipliers and the adder with the same word sizes. The conventional QAM modulator with the quadrature outputs requires four multipliers, two adders and sine/cosine memories. If the QAM modulator with the quadrature outputs is needed, the CORDIC based QAM modulator replaces four multipliers, two adders and sine/cosine memories.